Neurons communicate with target cells by the transient action of chemical substances called neurotransmitters. The goal of this research is to contribute to the understanding of these biochemical interactions between neurons in the central nervous system. The first objective will be to employ a transmitter screening procedure based on axonal transport to identify neurotransmitters in the vertebrate visual system. Initial studies will be done in lateral geniculate neurons of the rat. The approach will be to pressure inject small volumes of various high specific activity, radioactive transmitter precursors into regions of physiologically identified cell bodies. Radioactive substances that are axonally transported to specific neuron terminals will be identified biochemically, then tested to see if they fulfill various other criteria for neurotransmitter function such as localization, release, and physiological and pharmacological response. Autoradiographs of spinal cords injected with (3H) choline show that choline, probably in a lipid form, selectively accumulates over cell bodies, axons and principle dendrites of spinal motoneurons. A second objective will be to examine the metabolic significance of this accumulation and to determine if this could be used as the basis of an autoradiographic method for tracing cholinergic neurons. Biochemical investigations will be carried out to identify the observed compounds and their relationship to acetylcholine metabolism in motoneurons. Parallel studies will determine if other cholinergic neurons are similarly labelled and if this procedure is useful for tracing cholinergic neurons in the CNS. In vivo injections and explant cultures will be used to conduct these studies.